Dilute magnetic semiconductors: the role of Mn in GaMnAs(001) and Mn/GaAs(001) surfaces

The study of interfaces between magnetic materials and semiconductors is fundamental for the development of spintronics devices as spin injection blocks. For this reason the growth of Mn-based magnetic alloys on III-V semiconductors, in particular on GaAs is an interesting topic both from theoretical and technological point of views [1]. Moreover, the investigation of the GaMnAs(001) and the Mn/GaAs(001) surfaces is essential to understand the growth process of GaMnAs itself, a prototypical diluted magnetic semiconductor. In fact, the mechanism of interstitial Mn sites defects formation is not well understood as well as the structure of the GaMnAs(001)(1×2) surface; furthermore the Mn adsorption site could play a fundamental role [2]. As a last point it has been experimentally demonstrated that Mn acts as surfactant in the growth process of InAs quantum dots on GaAs [3]. In this thesis the growth and the characterization of the GaMnAs(001) and the Mn/GaAs(001) surfaces are reported. In the first case a Molecular Beam Epitaxy (MBE) growth of GaMnAs(001) with a subsequent in situ characterization by Reflectivity Anisotropy Spectroscopy (RAS), Electron Energy Loss Spectroscopy (EELS) and Scanning Tunneling Microscopy (STM) are presented. In the second case, starting from GaAs(001) grown by MBE, the surface was investigated after Mn deposition and subsequent low temperature annealing by in situ STM, X-ray Photoelectron Spectroscopy (XPS) and Photoelectron Diffraction (PED). The aim of this work is to understand the structure of 1×2 reconstruction of GaMnAs and to understand the mechanism of the Mn-As interaction and the behavior of Mn deposited on the GaAs(001) surface. The GaMnAs(001) surface reveals some electronic structures very similar to GaAs(001)c(4×4) [4] with an additional optical feature around 2eV attributed to the p-d hybridization between Mn (d) and As (p) [5], moreover the overall RAS spectra is very similar to GaAs(001)(1×2) surface [6] and to theoretical ab initio GaAs(001) ones [7] suggesting the presence of symmetric dimers in the GaMnAs(001) surface with a possible Mn adsorption site under the As-As dimers [8]. In the Mn/GaAs(001), after Mn deposition, LEED and RHEED patterns show that the c(4×4) reconstruction is destroyed and a faint (1×2) periodicity takes over, emerging more clearly, after the thermal treatment [9]. XPS measurements of As 3d and Mn 2p core levels before and after annealing demonstrate that the high reactivity of Mn with the surface As atoms is responsible for the surface disorder after Mn deposition. The sample annealing induces a rearrangement of the surface atoms. STM images demonstrate that the ordering of the surface responsible for the (1×2) periodicity, observed by LEED and RHEED, is characterized by structures compatible with the presence of As dimers. [1] M. Ramsteiner, et al. Phys Rev. B 66 (2002) 081304(R). [2] S.C. Erwin, A.C. Petukhov Phys Rev. Lett. 89 (2002) 227201. [3] S.P. Guo, et al. J. Crystal Growth 208 (2000) 799. [4] F. Arciprete, et al. Phys. Rev. B 68 (2003) 25328. [5] J. Okabayashi, et al Phys. Rev. B 59, R2486 (1999)-R2489. [6] M. J. Begarney, et al. Phys. Rev. B 62 (2000) 8092. [7] C. Hogan, E. Placidi and R. Del Sole Phys. Rev. B 71 (2005) 041308(R). [8] S. D. Thorpe, et al. to be published. [9] S. D. Thorpe, et al. Superlattices and Microstructures 46, (2009) 258-265.